1. Field of the Invention
This invention relates to a water-based paint which is produced by a phase inversion emulsifying process utilizing the surface-activating action of an acrylic resin incorporated in a paint resin solution, and to a process for its production. More particularly, this invention relates to a process for continuously preparing an emulsion type water paint comprising a carboxyl group-containing acrylic resin component, an epoxy resin component and a component of a curing agent resin for the epoxy resin.
2. Description of the Prior Art
In the production of cans for canning, it has been the previous practice to coat metallic blanks or cans themselves with various paints in order to prevent dissolving of metal in can contents or inhibit corrosion of the metal. In the case of not only deep-drawn cans produced from an uncoated metallic blank but also drawn cans or three-piece cans produced from a coated metallic blank, various paints are spray-coated and baked on can bodies or can closures in order to repair injuries caused to the coated film in the can-making process or to form top coats.
A paint comprising a combination of an epoxy resin and a curing agent resin and a vinyl-type paint are excellent in regard to adhesion to a metallic substrate, corrosion resistance, flavor characteristics and film processability. These paints exhibit good performance when coated in the form of an organic solvent solution. However, the solvent is dissipated in the working environment during spray coating, and problems of air pollution and environmental hygiene arise.
To eliminate these defects, water-based paints, namely paints in the form of an aqueous dispersion, have already been developed. A first type of such water-based paints comprises a paint resin rendered particulate by some means and dispersed in water using a surface-active agent or a water-soluble or hydrophilic resin as a dispersing agent (for example, Japanese Patent Publication No. 18076/1969). A second type is a paint obtained by reacting a paint resin having a functional group such as an epoxy resin with a resin having carboxyl groups such as an acrylic resin and neutralizing the modified resin with ammonia or an amine and causing it to self-emulsify in an aqueous medium (for example, Japanese Laid-Open Patent Publication No. 213718/1984).
However, the former type of water-based paint contains dispersed particles of the paint resin which generally tend to be large and non-uniform. Moreover, the water-based paint has low dispersion stability, and the properties of a coated film prepared from it is inferior to those of a film obtained from a solvent-based paint.
The later type of paint may have better dispersibility than the former type paint, but is greatly restricted in the composition of the paint resin. For example, in the case of an epoxy resin-type paint, it is difficult to secure a sufficiently high content of the resin curing agent, and the coated film cannot be fully cured. Consequently, the hardness and denseness of the coated film and its barrier property with respect to corrosive components cannot be improved to a satisfactory level.
Many coating methods have previously been proposed for forming good paint films using the above water-based paints.
For example, Japanese Laid-Open Patent Publication No. 133342/1977 described that a slurry-like paint is coated on an article, and until the coated film is subjected to drying under heating, it is maintained in an atmosphere having a temperature of not more than 100.degree. C. at which, in addition, the resin particles do not melt, a relative humidity of at least 80%, and an air speed of not more than 0.3 m/sec thereby to prevent cracking during film formation.
When a water-based paint containing emulsified resin particles is coated on the surface of a metallic substrate by such means as a roll coater, raised and depressed patterns such as ribs occur on the coated surface and remain there after drying and baking. Accordingly, the coated surface loses smoothness.
In the case of a paint in the form of an organic solvent solution, the resin solution has flowability and levelling of the wet coated film occurs during the time from coating to baking (generally called the period of setting). In the case of a dispersion-type water-based paint, such levelling is difficult presumably because the paint resin is present in the form of dispersed particles, the dispersing medium is water having a high surface tension and a high viscosity and evaporation of water proceeds considerably rapidly on the surface.
In a coating method using a dispersion-type water-based paint, no effective means has yet been known for alleviating raised and depressed patterns such as ribs occurring during coating and improving the levelness of the coated film.
A paint comprising an epoxy resin and a curing agent resin in combination or a vinyl-type resin is excellent in adhesion to a metallic substrate, corrosion resistance, flavor characteristics and processability of a coated film. However, it sometimes happens that a coated film formed from such a paint is not satisfactory in the smoothness and the completeness of covering. Namely, although occurrence of ribs can be prevented on the surface of the coated film formed from the above-mentioned paint, hard spots are readily formed on the surface, with the result that the surface smoothness is degraded and the covering of the substrate becomes incomplete.
We previously proposed a process in which an organic solvent solution containing a carboxyl group-containing acrylic resin component is prepared, ammonia or an amine and water are incorporated into the organic solvent solution to convert the carboxyl group in the acrylic resin to an ammonium salt or amine salt, and simultaneously, the resin component in the solution is self-emulsified to an o/w type emulsion. Namely, according to this process, a water paint is prepared by the phase inversion.
According to this process, however, since the resin solution has a considerably high viscosity, a large quantity of an organic solvent has to be used, and therefore, a large quantity of water should be incorporated for the phase inversion. Accordingly, the solid concentration in the formed aqueous emulsion is generally low, and a large quantity of the liquid should be handled. Therefore, the process is disadvantageous in that the dimensions of the apparatus should be increased and relatively large quantities of the solvent and water should be removed from the formed emulsion by evaporation.